Why does NI-9233 with IEPE microphone report 39dBA as minimum noise level?

Question

Hi,
I have been using an ACO Pacific 1/2" IEPE microphone and preamp (7052SYS) with cDAQ and a 9233 IEPE module. The ACO mic is very similar to the G.R.A.S. 46AE (50mV/Pa) but with a 20mV/Pa sensitivity. Using this set-up in a hemi-anechoic chamber, the SPL is reported as about 39dBA (using the Sound and Vibration standard  'sound meter' VI). Yet, the third party lab's signal analyzer reports the room at about 25dBA. I have partially verified the NI set-up with a 114dB 1kHz calibrator. The NI set-up indicates about 114.3dBA. So we are on the money there.
Is there a noise floor on the 9233 that I am dealing with? Any thoughts? Recently, my company has built its own hemi-anechoic chamber and we like the NI tools. We had hoped to use the newer 9234 with a cDAQ chassis since the 51.2kS/sec is sufficient for our measurement range of 20-20kHz. We already use cDAQ extensively here. 
Thanks,
-JD
Solved!
Go to Solution.

Answer 1

From the 9233 specs:
Integrating Noise density:
A-Weighted Power = ( A weighting power factor for bandwidth ) * (Bandwidth) * ( Noise Level )^2
A-Weighted Power = ( 0.53 ) * ( 20000 - 20 Hz ) * ( 400 nV / sqrt(Hz) / 20 mV / Pa )^2
A-Weighted Power = 4.26u Pa^2 rms
Leq (A-Weighted) = 2.06m Pa rms = 40.2 dB(A) ref 20u Pa
Calculating from Noise at full scale range:
FS = Voltage Range / Sensitivity
FS = ( 5 / sqrt(2) ) V rms / ( 20 mV / Pa ) = 176.8 Pa rms = 138.9 dB ref 20u Pa
Noise = 138.9 dB ref 20u Pa - 95 dB FS - 2.73 dB(A) = 41.2 dB(A) ref 20u Pa
So the fact that you are reading 39 dB(A) indicates that your 9233 noise is meeting specifications. There is still an issue that some effort needs to be made to better match the signal range to the input range of the 9233. You can add some external signal conditioning to apply pregain (20 dB) to the microphone output before the 9233. You also have other DSA hardware options that include input gain.
Doug
NI Sound and Vibration